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Anonymous Communication designs such as Tor build their security on distributed trust over many volunteers running relays in diverse global locations. In practice, this distribution leads to a heterogeneous network in which many versions of the Tor software co-exist, each with differing sets of protocol features. Because of this heterogeneity, Tor developers employ forward-compatible protocol design as a strategy to maintain network extensibility. This strategy aims to guarantee that different versions of the Tor software interact without unrecoverable errors. In this work, we cast protocol tolerance that is enabled by forward-compatible protocol considerations as a fundamental security issue. We argue that, while being beneficial for the developers, protocol tolerance has resulted in a number of strong attacks against Tor in the past fifteen years. To address this issue, we propose Flexible Anonymous Network (FAN), a new software architecture for volunteer-based distributed networks that shifts the dependence away from protocol tolerance without losing the ability for developers to ensure the continuous evolution of their software. We i) instantiate an implementation, ii) evaluate its overheads and, iii) experiment with several of FAN's benefits to defend against a severe attack still applicable to Tor today.